Vertical cavity surface emitting laser diodes (VCSELs) are laser diodes that generate light when driven with an electrical current. In general, an input electrical current drives the anode of the VCSEL causing the light emitting region, or aperture, of the VCSEL to emit a laser light beam. A VCSEL driver circuit is a circuit, typically an integrated circuit (IC), that receives a high-speed electrical voltage signal, converts this electrical voltage signal into an electrical current signal, and drives the anode of the VCSEL with a static direct current (DC) bias current that modulated with the converted high-speed current signal.
FIG. 1 illustrates a typical final output stage of a typical laser driver IC 1 that uses a 4.5-volt voltage supply. The anode 2 of the VCSEL 3 is driven with a bias current of 8 milliampere (mA) along with a modulation current of 10 mA (+/−5 mA). A modulator circuit 4 comprises an emitter-coupled NPN bipolar junction transistor (BJT) differential pair comprising first and second emitter coupled BJTs 5a and 5b, respectively, a load resistor 6 and current tail 7. The NPN BJT differential pair senses the polarity of the electrical input signal, VINP and VINM, and sinks either 0 mA or 10 mA from the output node 8 of the VCSEL driver IC 1. A P Metal Oxide Semiconductor Field Effect Transistor (PMOSFET) current source 9 sources a current equal to the sum of the bias current and one-half of the modulation current. PMOSFETs 11 comprise a current mirror. The difference between the current sourced by the PMOSFET current source 9 and the current sunk by the NPN BJT differential pair 5a, 5b is sent to the output node 8.
The PMOSFETs used in the PMOSFET current source 9 generally have a poor current-to-device dimension ratio compared to NPN BJTs for the same voltage across the device. Consequently, a PMOSFET current source presents more parasitic capacitance at its output terminals, resulting in poor high-bandwidth performance. In order to keep the parasitic capacitance contribution of the PMOSFETs to a minimum, they are typically permitted to have a large voltage across them, so that they can be kept small in size. Furthermore, the PMOSFET current source 9 is cascoded to keep the output resistance of the laser driver IC 1 high; which nearly doubles the required voltage headroom across the PMOSFET current source 9. These considerations mandate the need for a 4.5-volt voltage supply, which typically needs to be generated using a boost regulator (not shown) in a 3.3-volt system. The need for a boost generator has negative implications in terms of product area, power consumption, and cost.
Attempts have been made to replace the 4.5-volt voltage supply with a 3.3-volt voltage supply that remove the cascoded arrangement in the PMOSFET current source and increase the size of the PMOSFET current source to account for the loss of 1.2 volts of current source headroom. FIG. 2 illustrates a final output stage of a laser driver IC 12 that has been modified as such to use a 3.3-volt voltage supply by eliminating the cascoded arrangement in the PMOSFET current source 13 and increasing the size of the PMOSFET current source 13 to allow the supply voltage to be reduced. One disadvantage of the laser driver IC 12 is that the increase in the size of the PMOSFET current source 13 results in a significant increase in parasitic capacitance, which, in turn, results in an unacceptable loss in bandwidth.
Accordingly, a need exists for a laser driver circuit that is capable of operating at a low supply voltage and that is capable of achieving high-bandwidth performance.